added [UNIX Commands and Concepts] to [Unix]

free-programming-books.md

@@ -1766,6 +1766,7 @@ See also [LaTeX](#latex)
 * [Unix Toolbox](http://cb.vu/unixtoolbox.xhtml) - Colin Barschel
 * [UNIX Tutorial for Beginners](http://www.ee.surrey.ac.uk/Teaching/Unix/)
 * [Unix for Poets](http://www.stanford.edu/class/cs124/kwc-unix-for-poets.pdf) - Kenneth Ward Church (PDF)
+* [UNIX Commands and Concepts](http://www.cs.bu.edu/teaching/unix/reference/) - Robert I. Pitts
 
 
 ### Verilog